A major problem of the coating and finishing industry, both in terms of raw material usage and environmental effects, concerns the solvent components of paint. In a spray coating application of a resinous material, the resinous material is typically dissolved in an organic solvent to provide a viscosity suitable for spraying. This is required because it has been found that at each stage of the process for atomizing and conveying a resinous material in liquid form to a substrate, the liquid resists high speed deformation. Organic solvents are added to the resinous liquid because they have the effect of separating the molecules of resinous material and facilitating their relative movement making the solution more deformable at high speeds and therefore more susceptible to atomization. Substantial effort has been expended to reduce the volume of liquid solvent components in preparing high solids coating compositions containing above 50% by volume of polymeric and pigmentary solids. Nevertheless, most high solids coating compositions still contain from 15-40% by volume of liquid solvent components.
The problem with such a high volume content of liquid solvents is that during handling, atomization or deposition of the solvent coating compositions, the solvents escape and can become air contaminants if not properly trapped. Once the solvent coating is applied to a substrate, the solvents escape from the film by evaporation and such evaporated solvents also contaminate the surrounding atmosphere. In addition, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may also be created. Attempts at overcoming such environmental problems have proven to be costly and relatively inefficient.
One type of coating process which has been proposed as an alternative to those described above is the "Unicarb" process of Union Carbide Chemicals and Plastics Technology Corporation of Danbury, Conn. The Unicarb process includes the production of a high solids coating composition in which a substantial amount of the liquid solvent component has been removed and replaced with a non-toxic, supercritical fluid such as supercritical carbon dioxide. This coating composition is then sprayed onto a surface at which time the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating and to reduce drying time of the composition on the substrate. The term "supercritical" as used herein refers to a gas, which, above its critical pressure and critical temperature, has a density approaching that of a liquid material. Such supercritical fluid is relatively dense and behaves with solvent-like properties. Carbon dioxide is utilized in the Unicarb process because its critical temperature of 88.degree. F. and critical pressure of 1070 psi are within the operating parameters of most airless spray equipment used in coatings applications. The supercritical carbon dioxide and some solvent material, e.g., about two-thirds less than required in other high solids coating compositions, are intermixed with polymeric and pigmentary solids to form a coating composition having a viscosity which facilitates atomization through an airless spray gun. The supercritical carbon dioxide functions as a diluent to enhance the application properties of the paint.
Problems have been encountered in dispensing coating compositions containing supercritical carbon dioxide or liquified gas from conventional spray guns or other dispensers. It has been found that such dispensers permit the supercritical fluid or liquified gas to escape from solution, and/or convert to another phase, prior to discharge of the liquid coating material from the dispenser. Loss of supercritical fluid from the liquid coating composition makes it difficult to atomize the composition because its viscosity increases and also because less supercritical fluid is present to flash off or vaporize as the composition is sprayed to assist in atomization. As a result, the liquid coating tends to sputter or spit upon discharge from the spray gun, does not atomize and thus produces an inferior finish on the substrate to be coated.